At present, bidirectional digital data transmission between components of a system, such as occurs between a computer and a keyboard, is typically facilitated by transmitting and receiving circuitry configured in "open collector" configuration. As is conventional for this type of configuration, digital data from a port on each component is applied to a conductor coupling the components together, with a normal quiescent logic state of conductors of the data being a HIGH logic state, as represented by a voltage potential, such as +5 volts. In order to effect transmission of a digital data signal, the normally HIGH voltage potential is pulled to a LOW logic level by one of the components. This scheme is also characterized as active LOW, wherein components reading transmitted data are responsive to a falling voltage transition from a HIGH logic state to a LOW logic state.
While this scheme works well for relatively short communications links coupling the respective digital components, such as discrete conductors up to about 20 feet or so in length, long-distance coupling of open collector systems presents problems. Typically, such problems include signal attenuation by roll-off filters in the components, which are designed to reduce radiated electromagnetic interference, and attenuation by the long-distance conductors, which may reduce signal swing to a point where the digital signals will not reliably trigger digital devices. Also, where conductors conveying other types of signals, such as video signals, are located proximate the open collector conductors, induced noise therefrom may produce erroneous data bits. Further, where the respective open collector components are separated by distances on the order of thousands of feet, voltage offsets in ground potentials between the separated components may be such as to prevent one of the components from transitioning to a LOW logic level. This is particularly significant with TTL (transistor--transistor logic) systems wherein if the ground offset potential is above 800 millivolts, the upper limit for a TTL LOW logic level, unpredictable data transmission will result.
Applicant has devised several systems for long-distance digital data transmission, the most pertinent of which is patent application Ser. No. 07/736,568, filed on Jul. 26, 1991, now U.S. Pat. No. 5,323,420, by Robert R. Asprey and entitled "Circuitry For Regenerating Digital Signals in Extended Distance Communications Systems." Here, an input of a non-inverting buffer device having LOW and HIGH voltage thresholds with a hysteresis region therebetween is coupled to an open collector line and to a pull-up resistor, with an output of the buffer device coupled via a feedback resistor to the input. In this system, as a transition potential approaches one of the thresholds, additional current is provided to the conductor from the pull-up resistor and buffer device, which current serves to dampen noise as the transition potential crosses the threshold. Problems with it include lack of amplification, lack of threshold control, and the requirement of a common signal ground.
Accordingly, it is an object of this invention to provide a bidirectional, open collector, regenerative digital communications link that, in addition to eliminating the foregoing problems, may be extended thousands of feet and which in one embodiment may be powered by separate D.C. power supplies at respective ends of the communications link.